Master Switch

"Michael" wrote in message
ups.com...
....snip...

It is the avionics master is off during engine start to keep spikes
from getting to the avionics. I suspect that's the cause of the OWT.
In airplanes lacking an avionics master, optimal procedure is to shut
down the avionics prior to engine shutdown and turn them on only after
engine start. In cars this is accomplished by means of an automatic
contactor, but in the aviation world we're not up to that level of
technology.


I am reading from the generic POH for 1976 Cessna 172, section 7, Airplane
and Systems Descriptions, Electrical System:

quote:
Both sides of the bus are on at all times except when either an external
power source is connected, or the *starter switch is turned on*; then a
power contactor is automatically activated to open the circuit to the
electronic bus. Isolating the electronic circuits in this manner prevents
harmful transient voltages from damaging the transistors in the electronic
equipment.
:unquote.

I take it back. I guess on some airplanes we ARE up to the basic
automotive level of technology.

I've never owned a Cessna, and the Pipers I've owned had no such
cutout.

Michael

"JohnH" wrote in message
...
Jose wrote:
On an aircraft with separate "battery" and "alternator" switches,
should you turn on BAT and leave ALT off during engine start, or
should you turn them both on prior to start?

I leave alt off during start, to prevent a power surge from going
through the alternator.

What would be the source of this surge? If it's such a problem, why don't
automobiles have alternator switches?


Many do but it is automatic. They don't power the buss for a second or two
after start to allow the alternator circuit to stabilize.

"Icebound" wrote in message
...
I am reading from the generic POH for 1976 Cessna 172, section 7, Airplane
and Systems Descriptions, Electrical System:

quote:
Both sides of the bus are on at all times except when either an external
power source is connected, or the *starter switch is turned on*; then a
power contactor is automatically activated to open the circuit to the
electronic bus. Isolating the electronic circuits in this manner prevents
harmful transient voltages from damaging the transistors in the electronic
equipment.
:unquote.

By 1981, though, the 172P POH says: "The primary bus is on anytime the
master switch is turned on, and is not affected by starter or external power
usage. Both bus bars are on anytime the master and avionics power switches
are turned on."

Perhaps Cessna decided that the electric-cutout switches created an
unnecessary risk of accidental loss of electricity.

--Gary